It will also be important to point out that the integer \(n\)  in the quantization of \(\rho _{xy}\)  was eventually understood as a topological invariant,
sometimes referred to as a Chern number [1]}. This fact explains the robustness of the quantized plateaux in spite of
variable details of the sample, in particular the realization of disorder, which varies from sample to sample.
It is also known that the transitions between plateaux are infinitely sharp at zero temperature, and for the most part we assume we are in this situation.
We wish to also mention that the IQHE also exists in models without Landau levels [2]}.
